1. Field of the Invention
This invention relates to an optical system of an optical information recording/reproducing apparatus in which a semiconductor laser is used as a light source. The invention further relates an objective optical system and a chromatic aberration correcting element suitable for use in the optical system.
2. Description of the Prior Art
An optical system of an optical information recording/reproducing apparatus such as optical disk apparatus, etc. is generally shown in FIG. 52. Such an optical system includes a light source portion 10 for emitting a generally parallel luminous flux, an objective optical system 20 for converging (focusing) the luminous flux emitted from the light source 10 onto an optical disk OD, a beam splitter 30 for splitting the luminous flux disposed between the light source 10 and the objective optical system 20 and adapted to split the luminous flux reflected by the disk, and a signal detecting optical system 40 for receiving such split luminous flux.
The light source portion 10 has a semiconductor laser 11, a collimator lens 12, and a beam shaping element 13.
The objective optical system 20 includes an objective lens 21, and a mirror 22, which are disposed within a head 50 which is slidable in the radial direction of the optical disk. Also, the objective lens 21 is mounted on an actuator (not shown) disposed within the head 50 and designed such that the objective lens 21 can be finely moved at least in the optical axis direction thereof so that an out-of-focus caused by warping of the disk, etc. can be corrected.
The signal detecting optical system 40 has a beam splitter 41, a tracking signal detecting system 42, and a focusing signal detecting system 43, and is adapted to read information recorded in the disk and error signals of the tracks by light reflected from the optical disk OD.
Incidentally, an emitting light wavelength of the semiconductor laser used as a light source is shifted as a result of changing of output power and/or temperature. Because of the foregoing reason, when the chromatic aberration of the optical system is not yet corrected, the position of a light converging point is varied into a shift of the wavelength.
When the light converging position is not coincident with the recording surface of the disk, there is a high possibility that incorrect writing and reading will be performed.
However, out of focus due to comparatively gradual change of wavelength caused by change of temperature or the like is automatically corrected by the aforementioned focusing servo when the collimator lens is corrected for chromatic aberration and for change of temperature.
However, at the time when data is written, an oscillation wavelength of a semiconductor laser is instantaneously shifted by several nm between a region where the temperature is increased and a region where the temperature is not increased. The out of focus caused by such radical shift cannot be corrected by the above-mentioned focusing servo.
Therefore, especially when writing is performed, correction of the chromatic aberration of the objective optical system is important.
An optical system in which the objective lens itself is corrected in chromatic aberration is disclosed in, for example, Japanese Patent Early Laid-Open Publication No. SHO 63-10118; Japanese Patent Early Laid-Open Publication No. SHO 60-232519; and Japanese Patent Early Laid-Open Publication No. SHO 58-72114.
However, the lens of the Japanese Patent Early Laid-Open Publication No. SHO 63-10118 is of a three component structure including an aspherical lens, while the lenses of the Japanese Patent Early Laid-Open Publication No. SHO 60-232519 and the Japanese Patent Early Laid-Open Publication No. SHO 58-72114 are of four component structures of glass lenses. Accordingly, these lenses are heavy in weight compared with lenses which are not corrected in chromatic aberration, and a load imposed to a movable actuator is large.
As an objective lens for an optical disk apparatus is moved at a high frequency for the purposes of focusing and tracking, it is an important requirement that the objective lens is made compact in size and light in weight in order to reduce the load on the actuator.
Also, Japanese Patent Early Laid-open Publication No. Sho 62-269922 discloses an optical system for correcting the chromatic aberration of an objective lens by excessively correcting the chromatic aberration of a collimator lens. With this construction, it is necessary to excessively correct even a focusing error detecting optical system because otherwise, out of focus is generated or caused by the focusing servo.
However, the correcting amount of the chromatic aberration of the focusing error detecting optical system is proportional to a second raised power of the ratio M, between a focal length of a condenser lens of this optical system and a focal length of the objective lens. Therefore, in an ordinary optical disk apparatus taking a value of about M=10 in view of the size of a light receiving element ,it is difficult to design a system such that the condenser lens can sufficiently correct the chromatic aberration.